Concrete is extensively used in the construction industry. The compressive strength of concrete and its supporting capabilities are directly related to the amount of water that is utilized in proportion to the amount of cement that is mixed with the water (water/cement ratio). It would be beneficial to the building trade to be able to determine the compressive strength of concrete in a non-destructive manner, i.e., using a technique that does not cause damage to the concrete sample under inspection.
Technologies have been developed for detecting the strength of concrete and include those generally identified as: pulse velocity method, surface hardness, penetration, pull-out, break-off and maturity techniques. Some of these techniques are not completely non-destructive since some damage to the concrete results from using them. Furthermore, such techniques are limited because the accuracy and reliability of the results are not well established.
Microwave technology has also been proposed for use in analyzing concrete materials. In that regard, concrete can be characterized, like other insulating materials, as having a dielectric constant (.epsilon.). The value of the dielectric constant for a concrete sample has been found to be a function of the water/cement (w/c) ratio. In determining the dielectric constant, a reflection coefficient (R) is determinable using the ratio of a reflected microwave from the concrete sample to the transmitted microwave that is incident upon the sample. The value of the reflection coefficient (R) is a complex quantity that includes a magnitude (.GAMMA.) and a phase (.phi.). It has been observed that the magnitude (.GAMMA.) of the reflection coefficient varies as a function of the w/c ratio. More particularly, the magnitude of the reflection coefficient decreases as the w/c ratio increases. Since the w/c ratio is indicative of, or directly related to, the compressive strength of concrete, the reflection coefficient magnitude is useful in analyzing a concrete structure in connection with determining strength related information.
It would therefore be advantageous to provide an apparatus and method that automatically determines the water/cement ratio and/or the compressive strength of any concrete sample using microwave signals. Such microwave technology would permit non-destructive inspection or testing and would rely on the magnitude of the reflection coefficient in subsequently correlating the determined reflection coefficient magnitude with desired concrete strength related information.